Just 1.4 billion years after the Big Bang, the galaxy MXDFz4.4—one of the universe’s smallest galaxies at only 100 times the size of the Milky Way—emitted a powerful burst of ionizing light through the neutral hydrogen that enveloped it. This phenomenon provides astronomers with unprecedented insights into the reionization process of the universe.
This diagram depicts the galaxy MXDFz4.4 as it existed 1.4 billion years after the Big Bang. Image credit: NASA/ESA/Leah Hustak, STScI.
MXDFz4.4 thrived during the Reionization Era, a transformative period in cosmic evolution.
In the early stages post-Big Bang, the gas surrounding stars and galaxies obstructed high-energy ultraviolet light.
Gradually, this gas transitioned to a transparent or ionized state over hundreds of millions of years, rather than a sudden change.
“It was previously believed that observing galaxies like this would be impossible,” stated Dr. Ilias Guvarts, a postdoctoral researcher at the Space Telescope Science Institute.
“Researchers anticipated that the dense ‘fog’ of neutral hydrogen would obscure the ionizing light.”
“Hubble not only detected this light but also uncovered intriguing details about the galaxy’s features.”
Visible-light images from Hubble reveal that intense bursts of young stars have cleared MXDFz4.4 and its vicinity. Image credits: NASA / ESA / CSA / STScI / Ilias Goovaerts, STScI / Marc Rafelski, STScI & JHU / Anton Koekemoer, STScI / Alyssa Pagan, STScI.
“Although astronomers have identified many galaxies from this period in cosmic history, MXDFz4.4 is unique as it is the only one to have emitted ionizing photons,” Dr. Mark Rafelski from the Space Telescope Science Institute noted.
Long exposure observations by Hubble showed that a massive young star within MXDFz4.4 was responsible for the ultraviolet light filtering through the primordial universe.
These stars emerged simultaneously over the past few million years and are closely compacted together.
Interestingly, MXDFz4.4, though about 100 times smaller than the Milky Way, is forming stars at a rate 10 times greater.
“The presence of numerous young, hot, massive stars in a compact area aids in dispersing the opaque gas,” explained Dr. Guberts.
The research team’s paper was published on June 23, 2026, in the Astrophysical Journal.
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Elias Guvaarts et al. 2026. MXDFz4.4: First test of LyC emitters 250 million years after the epoch of reionization and the Lyα form as a tracer of LyC escape at high redshifts. APJ 1005, 34; doi: 10.3847/1538-4357/ae75b0
Source: www.sci.news
